Oscillator and method of making for atomic force microscope and other applications

ABSTRACT

A method for making a device which includes an oscillator, which includes depositing on silicon or other first material a layer of silicon nitride or other second material, forming in the first material a support structure and the oscillator, including applying an etchant which is selective for the first material to etch all the way through the first material and leave the second material substantially unetched to thereby form the at least one flexible hinge of the second material.

This is a divisional of co-pending prior application Ser. No.11/506,970, filed Aug. 18, 2006, and the priority of U.S. provisionalpatent application Ser. No. 60/709,930, filed Aug. 19, 2005, is herebyclaimed. The disclosures of both of the aforementioned prior andprovisional applications are hereby incorporated herein by reference.

The present invention relates generally to the investigation,measurement, manipulation, or otherwise sensing of the topographicfeatures of a surface or object, particularly at the microscopic oratomic level, and includes devices having oscillators for otherpurposes. An example of an application of the present invention is asensor for an atomic force microscope (which is also referred to hereinas “AFM”).

An atomic force microscope usually utilizes a sharp stylus or tip whichis caused to move over the surface or object under investigation orotherwise detect the surface or object to, for example, map the physicalcontour and/or the softness or hardness spectrum (derived from energylevel) over the surface. It can even manipulate samples such as thecutting of a chromosome (by increasing the force on the sample and usinga very sharp tip and very stiff cantilevered member supporting the tip).The stylus or tip is supported by a stiff beam or pad (or is integrallypart of the beam) which is compliantly supported by a hinge attached toa support structure so that it acts (moves) as a spring. Thus, theorientation or deflection of the cantilevered beam or pad changes inrelation to topographic changes in the surface or object underinvestigation as it is moved over the surface or object. The beamdeflection is monitored typically by a laser beam reflected off the padonto a position sensitive photodetector, as more particularly describedhereinafter with reference to FIG. 2. Thus, the atomic force microscopemay be said to be more akin functionally to a record player reading thetopography of a record than to the traditional notion of a “microscope.”The atomic force microscope belongs to the family of microscopes knownas scanning probe microscopes. For example, the scanning tunnelingmicroscope has a conductive tip for sensing electrical properties.

Tip properties define minimal resolution of surface topography.Physically, the cantilevered beam may be considered to be an oscillator.Since thermal noise is inherent in mechanical systems, the force ordisplacement resolution of the cantilevered beam is limited by itsmovement due to thermal noise. Soft cantilevered beams (those whosehinge or hinges are more compliant) are required to measure smallforces.

The AFM is of particular importance in the field of biology since it isone of the few instruments than can be operated to measure mechanicalproperties in salt water, life's preferred medium, as well as otherfluids, and can be used for measurements on live samples under water.Moreover, the AFM can be used in liquid for scanning at atomicresolution, and it allows measurements of forces in the regimes ofsignificance such as single molecule force spectroscopy.

In liquid, the cantilever response is hindered due to viscous drag andcoupled liquid mass, which results in a smaller operational speed limitand decreases force resolution. A “cantilever” is defined herein asincluding the tip or other substrate engagement means and a hinge orhinges cantileverly connecting the tip or other substrate engagementmeans to a support structure (chip) and further includes any beam or padto which the tip may be attached, integrally or otherwise. Since theimpact of drag on an object moving in fluid is a function of theobject's speed and size, the cantilever size should be minimized. Thus,AFM cantilevers for operation in salt water (or for other sensitiveoperation outside vacuum) should be small in size.

The first cantilevers had beams made from a gold foil with an attachedsmall diamond tip. Later, silicon micromachining technology was used tomake cantilevers in parallel production with well-defined mechanicalproperties. Presently, AFM cantilevers are made from a variety ofmaterials, the more common of which are silicon (Si), stoichiometricsilicon nitride (Si₃N₄), low stress silicon-rich silicon nitride (SiN),and silicon dioxide (SiO₂).

A typical cantilever having a tip and a hinge (microcantilever spring)connecting the tip to a support chip is illustrated in FIG. 1 ofGustaffsson et al, “Scanning Force Microscope Springs Optimized forOptical-beam Deflection and with Tips Made by Controlled Fracture,” J.Appl. Phys., vol. 76(1), 1994, pp 172-181. Since the spring curvaturethereof can distort the measurements, the tip is typically located on amore rigid beam or pad from which the laser beam is deflected, such asillustrated in FIG. 3 thereof.

Silicon cantilevers have typically had hinges which are generally thick(greater than 1 micron) and therefore not very soft (compliant).

Soft cantilevers (having soft or very compliant hinges) have usuallybeen made from silicon nitride, because low-stress highly uniform filmsof Si₃N₄ can be grown very thin (less than 1 micron). Such softcantilevers are large (having hinges on the order of hundreds of micronslong) and hence have low resonant frequencies, particularly in liquids.The mechanical compliance for cantilever hinges is a cubic function ofthe length and an inverse of the cubed thickness thereof. Thus, softcantilevers can be made by elongation or thinning. Typically, highlycompliant cantilevers are made long (the hinges being on the order of300 microns long). Such cantilevers with long hinges have large surfaceareas and are highly damped in liquid.

To increase reflectivity, cantilever beams or pads have often beencoated with a thin layer (40 to 50 nanometers) of aluminum or gold.

It is desirable to provide cantilevers of smallest possible dimensions(desirably on the order of the diameter of the measuring laser beam, forexample, about 20 microns) so that their optical gain and the frequencyresponse of the probes are maximized.

Silicon nitride cantilevers have been built with hinges as thin as 86nanometers and as short as 27 microns. They have shown the promise ofallowing fast and quiet AFM imaging. However, there are some drawbacksto such small cantilevers. Due to their small size, there is a limitedarea of access to the sample (surface under investigation) and to thepad for receiving the optical (laser) beam. Therefore, AFMs that usesuch small cantilevers undesirably require specialized optics and cannottherefore be integrated into off-the-shelf AFMs. Also, when a smallcantilever hinge bends, it undesirably attains higher curvature than alonger cantilever hinge with the same spring constant (as in the case ofa bendable mirror). Additionally, due to the asymmetry of such typicalthin-hinged cantilevers, their hinges often undesirably curl during theproduction process as a response to intrinsic film stress and thermalstress.

For scanning or sensing soft samples, it is considered desirable to usecantilevers with soft (compliant) hinges since they minimize sampleperturbation and maximize sensitivity. It is also considered desirablethat the cantilevered member which supports the tip as well as thesupport structure be of adequate stiffness so that the entire cantileverdoes not undesirably flex.

Cantilevers with thin and therefore soft hinges are difficult to makefrom silicon unless expensive E-beam lithography is used, since it isdifficult to precisely control the silicon etch process to leave asufficiently thin layer.

In a Cornell Nanoscale Facility internet publication (NationalNanofabrication Users Network) entitled “Hinged Atomic Force MicroscopyCantilevers” (CNF project 883-00), Mechanical Devices, pp 174-175,published on the internet in 2003, Applicants discussed a fabricationprocess for their hinged cantilever (shown in FIG. 3 thereof as having abeam, support structure, and a thin hinge connecting the beam andsupport structure). In that publication it is disclosed that the“fabrication process involves three main steps: 1) backside die etch; 2)direct tip fabrication; and 3) lever and hinge fabrication. Dies aredesigned to be supported by one edge, defined on the backside by aself-terminated KOH etch. We use ‘dog-ears’ for compensation of exposedcorners during the backside KOH etch. On the topside, we can fabricatethe tip directly, either by KOH or SF6 liftoff. The tip can beoxidation-sharpened if an atomically-sharp tip is desired. Siliconnitride hinges can survive multilevel processing because they areprotected on both sides by silicon oxide until the very last releasestep.” It is also disclosed that “dual-hinge & torsion levers can befabricated using this process with modification only at the CAD level.”The hinges are disclosed as being made of silicon nitride, and backsideKOH is disclosed for fabrication of dies and thin silicon membranes. Seealso an earlier similar publication published on the internet in 2001 bythe Applicants entitled “Hinged Atomic Force Microscopy (AFM)Cantilevers” (Project 591-96), Biology & Chemistry, pp 16-17, wherein itis disclosed that the stiff back-beam of the cantilever is made ofsilicon and that the hinge region is a thin silicon nitride film.Applicants also published a similar publication on the internet in 2005entitled “Torsional AFM levers for Sensitive measurements in Liquid,”(Project 883-00), Mechanical Devices, pp 170-171. These publications donot disclose all the steps of the process referred to therein, and theycertainly do not disclose the process or cantilever adequately to enableone of ordinary skill in the art to make the cantilever.

As previously discussed, for biological applications, it is importantthat a cantilever function well in liquids such as salt water.Undesirably, the heretofore described hinged asymmetric cantilever (suchas shown in the previously cited Gustaffsson et al article as well as inthe previously cited Cornell publications) warps when it becomeshydrated in a liquid and also warps with temperature or contamination orother environmental perturbations. Moreover, while such a hingedcantilever provides compliance and sensitivity vertically (called hereinthe “z axis,” which is normal to the hinge), it has little or nocompliance or sensitivity laterally (called herein the “x axis”) of thehinge.

It has been suggested that the member or beam supporting the tip beconnected to the support structure through a pair of flexible membersacting as torsion springs, about which the beam can rotate. It isfurther suggested that such a torsion cantilever be made from amorphoussilicon nitride, and it is implied that single-crystal silicon couldalternatively be used. See FIG. 6d of the previously cited Gustaffssonet al article. Also see Miller et al, “Microelectromechanical ScanningProbe Instruments for Array Architectures,” Rev. Sci. Instrum., vol.68(11), 1997, pp 4155-4162; and Miller et al, “Proc. SPIE, 2640, 45,1995. As previously discussed, it is difficult to make such cantileversout of silicon or silicon nitride so that the torsion springs are thinenough unless expensive E-beam lithography is used.

What is measured by an AFM is traditionally the force on the tip of thecompliant cantilever. Since the AFM instrument is in the dimensionalrange of 10 cm. or greater, extraneous movements will undesirablydeflect the tip while it is otherwise being deflected by the sample.Also, environmental vibrations and the like can shake (deflect) thecantilever relative to the object being investigated. Such unwanteddeflections create low frequency noise. It is considered desirable toremove such unwanted noise in order to increase measurement accuracy andprecision while providing the ability to conduct longer experiments. Itsremoval also reduces demands on microscope design since drift in the zaxis is desirably reduced.

In order to remove such unwanted noise, it has been suggested to measureand subtract from the measured tip movements the substrate movements,utilizing two separate cantilever sensors sitting side-by-side (possiblyon the same die) and each detected independently to split the tasks ofsubstrate and sample position measurement. See Altmann et al, “MultipleSensor Stabilization System for Local Probe Microscopes,” Rev. Sci.Instrum., vol. 72, 2001, pp 142-149; and U.S. Pat. Nos. 6,545,492;6,798,226; and 6,583,411. Other patents which may be of interest in thisregard are U.S. Pat. Nos. 5,515,719 and 6,819,822.

The above system of Altmann et al requires dual laser beams for theindependent detection of the sensors. Alignment thereof undesirably istedious and difficult, and such a system also undesirably requires aspecialized microscope.

Other patent references which may be of interest to the presentinvention are U.S. Pat. Nos. 5,386,110; 5,874,668; 6,016,693; 6,066,265;6,291,140; 6,690,008; 6,734,598; 6,864,481; and 6,867,443 and U.S.patent application publications 2001/0049959 and 2006/0005614. Thesepatent references and other patent references discussed herein areincorporated herein by reference.

It is accordingly an object of the present invention to provide animproved torsion cantilever wherein the properties of the cantileveredmember and support structure and of the torsion bars may be tailored totheir respective specific requirements.

It is a further object of the present invention to provide a torsioncantilever wherein the cantilevered member and support structure are ofadequate stiffness so that the entire cantilever does not undesirablyflex while the torsion bars or hinges have the flexibility or softnessof the desired sensitivity.

It is yet another object of the present invention to provide a method ofmass production of micro-mechanical oscillators with flexible hingeswith a wide variety of geometries and mechanical properties (sizes,spring constants, etc.), including custom shapes and modes of motion, ona single die.

It is a further object of the present invention to use standardmicro-fabrication equipment, without involvement of advanced techniquessuch as E-beam lithography, for such a mass production method.

It is another object of the present invention to use inexpensive siliconwafers (not, for example, more expensive SOI wafers) for such afabrication process.

In order to provide an improved torsion cantilever wherein the torsionbars may be made thin enough to provide the desired compliance(softness) but without expensive E-beam lithography, in accordance withthe present invention, torsion members are part of a layer of siliconnitride or other suitable material which is applied to the cantileveredbeam and support structure made of silicon or other suitable differentmaterial.

In order to mass produce composite material micro-mechanical oscillatorswith torsion bars or other flexible hinges with a wide variety ofgeometries and mechanical properties (sizes, spring constants, etc.),including custom shapes and modes of motion, on a single inexpensivewafer using standard micro-fabrication equipment, in accordance with thepresent invention, a layer of a second material is deposited on a wafercomposed of a first material, and the support structure and theoscillator are formed in the wafer, including applying an etchant whichis selective for the first material to etch all the way through thefirst material and leave the second material substantially unetched tothereby form the at least one flexible hinge of the second material. Theoscillators are made to be easily removed from the silicon wafer, to besupported on a large silicon die which can be used for manipulation, aremade of three standard micro-fabrication materials (silicon nitride,silicon dioxide, and silicon), and are provided to have very thinflexible hinges and thicker very stiff silicon support structures andcantilevered members.

The above and other objects, features, and advantages of the presentinvention will be apparent in the following detailed description of thepreferred embodiment(s) thereof when read in conjunction with theappended drawings in which the same reference numerals denote the sameor similar parts throughout the several views.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective schematic view of a handle chip (in partialview) containing a hinged sensor made in accordance with the process ofthe present invention.

FIG. 2 schematic view of an atomic force microscope (AFM) which employsthe sensor.

FIG. 3 is a perspective schematic view of a handle chip (in partialview) containing a sensor which embodies the present invention.

FIG. 4 is a front view thereof.

FIG. 5 is a schematic plan view of a sensor in accordance with analternative embodiment of the present invention.

FIG. 6 is a schematic plan view of a sensor in accordance with anotheralternative embodiment of the present invention.

FIGS. 7 a to 7 s are schematic views, taken along lines 7 s-7 s of FIG.1, of a sequence of steps applied to a wafer in making the sensor ofFIG. 1.

FIG. 8 is a schematic representation of a template (mask) for formingthe handle chip, illustrated in dashed lines, of FIG. 1.

FIG. 9 is a schematic bottomside view of the handle chip of FIG. 1.

FIGS. 10, 13, 14, 15, and 17 are schematic plan views of a sensor inaccordance with additional alternative embodiments of the presentinvention.

FIG. 11 is a schematic view of a wafer illustrating the formationtherein of a multitude of the handle chips.

FIGS. 12 and 16 are views of the sensors of FIGS. 10 and 15 respectivelyillustrating how they work.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring to FIG. 2 of the appended drawings, there is illustratedgenerally at 30 an atomic force microscope (AFM) for use in sensing andmeasuring or otherwise investigating the three-dimensional topology orsurface properties such as contour of the surface 32 of an object orsample 34, for example, cells or other biological material. The AFM 30includes a sensor 10 having a cantilevered member 16 to which a pointedor sharp tip or stylus 18 is integrally or otherwise suitably mounted,wherein the tip 18 is movable over the surface 32 (or otherwise detectsthe surface or object such as a molecule) causing the cantileveredmember 16 to cant as the contour of the surface 32 changes, thus sensingchanges in surface contour or otherwise sensing the surface or object.With reference to sensing, the term “surface,” as used herein and in theclaims, is meant to include objects such as cells under investigation.The cantilevered member 16 and tip 18 define a probe. It should howeverbe understood that, while the usual applications will utilize a tip, thepresent invention does not require that the probe have a tip. A thincompliant hinge 12 connects the cantilevered member 16 to supportstructure or handle chip 14.

As seen in FIG. 9 as well as FIG. 1, the bottomside (side opposite thetip) of support structure 14 has a thin portion 33 extending over adistance, illustrated at 68, from the hinge 12, and the supportstructure 14 is thereafter sloped, as illustrated at 19, to define amuch thicker portion or handle 23 for supporting operational devices formoving and monitoring the sensor 10. The operational devices aresupported and used in accordance with principles commonly known to thoseof ordinary skill in the art to which this invention pertains. Thehandle chip 14 may contain a plurality (3 shown) of sensors 10 extendingfrom the front end thereof. The plurality of sensors 10 may be redundantor may be in a variety of types/sizes for a variety of tasks. The setback 68, which, for example, may be about 150 microns, and the slanting19 (which is achieved by suitable etching which also provides thehereinafter described pre-etching at 25) are provided to keep the chip14 and its shadow from interfering with the laser beam 48 (describedhereinafter). For example, the handle portion 23 may have a thickness inthe neighborhood of 400 to 450 microns (the typical thickness of a waferfrom which the sensor is made) and a width and length of severalmillimeters each. An array of handle chips 14 each containing aplurality of sensors 10 may be formed in a wafer, illustrated at 27 inFIG. 11 (the scale being too large in FIG. 11 for the sensors to beshown). The wafer is etched through along the ends and one side of thechip 14, as illustrated at 29, wherein the chip 14 is connected to thewafer only along one edge 25. Attachment edge 25 is desirably pre-etchedto allow the chip 14 to be easily broken away from the wafer for use.Accordingly, the thickness of the handle chip 14 (except for the thinnerportions 33) is generally equal to the thickness of the wafer from whichit is made. It should of course be understood that the support structure14 and cantilevered member 16 may be otherwise suitably sized and shapedas required or desired for each application, for example, as hereinafterdescribed with respect to FIGS. 3 and 4.

As is well known in the art, a piezotranslator 42 positions the supportstructure 14 while the displacement or canting of the cantileveredmember 16 is monitored by what may be called an “optical lever”, i.e., alaser beam 48 provided by a suitable laser device 50 and reflected fromthe back or bottomside surface (the surface opposite the surface onwhich the tip 18 is located) on the cantilevered member 16 onto apositron sensitive photodetector 52, as illustrated at 54. Optic beamsother than laser may be alternatively employed. The piezotranslator xand y positions and velocities and the z-position are controlled by ageneral purpose computer 56, as illustrated at 58 and 62 respectively,and may be modulated in feedback, as illustrated at 60 and at 58, 62respectively. The x, y, and z axes are illustrated at 17, 19, and 15respectively in FIGS. 1 and 3. The laser beam reflection 54 providesinformation about the canting of the cantilevered member 16 in responseto the work performed by the surface 32 on the tip 18 being rasteredbeneath it. When operated in feedback, software feedback loop 60controls the piezotranslator 42 to minimize the bending of thecantilevered member 16 to maintain a more controlled force on thesurface 32. While a variety of operational modes of AFM exist, such ascontact (repulsive), non-contact (attractive), and intermittent contact,they can be roughly divided into the categories of constant force andconstant height. In the constant force mode, the support structure 14 ismoved up and down to maintain a constant deflection (force) at the tip18. In a constant height mode, the mean position of the supportstructure 14 is fixed, and the force field is sampled by the tipdeflection. The above principles of the AFM are well known to those ofordinary skill in the art to which the present invention pertains. Whilethe sensor of the present invention is described in connection with anAFM, it should be understood that it may have other suitableapplications such as, for example, other types of scanning probemicroscopes.

Referring again to FIG. 1, the pad 16 may be sized to accept the typical(20 micron diameter) optical beam waist of commercial AFMs. Minimizationis limited only by the optical resolution of contact photolithography,i.e., about 0.5 micron. An additional factor in determining pad lengthis the optical gain, i.e., shorter pads have higher gains. Thus, shorterprobes may be made stiffer and still retain high sensitivity.Accordingly, the cantilevered member 16 may have a length and width inthe neighborhood of, for example, 10 to 30 microns each. Thecantilevered member 16 may be otherwise suitably sized and shaped and ofcourse made smaller. The pyramidal or conical tip 18, whose sharp point21 engages the surface 32, is preferably integral to the cantileveredmember 16 (for ease of manufacture in accordance with the processdescribed hereinafter) but may be otherwise suitably attached.

In accordance with the hereinafter described process of the presentinvention, the thin compliant hinge 12 is a part or portion of a thinlayer or coating 20 of material which substantially spans (but need notspan entirely) the length and width of the sensor 10, i.e., it isapplied (attached) to the back or bottom sides (opposite the side onwhich the tip is located) of the cantilevered beam 16 and supportstructure 14, and the layer portion 12 extends therebetween to definethe compliant hinge.

In order to more easily and inexpensively make the sensor, includingcontrol of hinge thickness, as discussed more fully hereinafter withrespect to FIGS. 7 a to 7 s, the hinge material is different from thematerial of which the member 16 and tip 18 and structure 14 are made.The member 16 and tip 18 and the supporting structure 14 are made ofsilicon or other suitable material of a thickness selected to besufficient to provide the desired stiffness, while the hinge material isLPCVD (low pressure chemical vapor deposition) silicon nitride or othersuitable material, such as, for example, polyimide, gold, SU8 (which isa photolithographic plastic), or other suitable plastic, whose thicknessis selected to be sufficiently thin to provide the desired compliancy(softness). Preferably, the thickness of the cantilevered member 16 aswell as the support structure 14 are the least that is suitablyfunctional and has sufficient rigidity. In order to have sufficientrigidity, the thickness of each of the silicon cantilevered member 16and silicon support structure portion 33 is at least about 1 micron, forexample, the member 16 and thin support portion 33 may have a thicknessof about 5 microns (and of course may be different if made of othermaterials). In order for the hinge 12 to be sufficiently compliant, thesilicon nitride layer 20 of material and thus the hinge 12 preferablyhas a thickness less than about 100 nanometers, for example, about 20 to50 nanometers (and of course may be different if made of othermaterials). Thus, the cantilevered member 16 and support structureportion 33 are on the order of 100 times thicker than the hinge 12 so asto achieve the desired compliancy for the hinge 12 while providing thedesired stiffness to the rest of the cantilever.

Referring to FIGS. 3 and 4, there is shown generally at 38 analternative embodiment of a sensor, which is attached to a handle chip14 similarly as described for sensor 10. A thin support portion 72,similar to support portion 33, has a pair of spaced parallel arms 70protruding from its front end, with the set-back 68 defined rearwardlyfrom the arms 70. Each of the arms 70 has a length and width of, forexample, about 150 and 10 microns respectively. A cantilevered beam 36,similar to beam 16, is attached between the arms 70 by hinge members 76,which are described in greater detail hereinafter. It should beunderstood that, in accordance with the present invention, thecantilevered beam 36 (or any other of the cantilevered beams disclosedherein) need not be part of a sensor. For example, it may be anoscillator for an accelerometer.

The pad 36 may be sized to accept the typical (20 micron diameter)optical beam waist of commercial AFMs. Minimization is limited only bythe optical resolution of contact photolithography, i.e., about 0.5micron. An additional factor in determining pad length is the opticalgain, i.e., shorter pads have higher gains. Thus, shorter probes may bemade stiffer and still retain high sensitivity. Accordingly, thecantilevered member 36 may have a length and width in the neighborhoodof, for example, 20 microns each. The cantilevered member 36 may beotherwise suitably sized and shaped and of course made smaller. Thepyramidal or conical tip 40, whose sharp point 74 engages the surface32, is preferably integral to the cantilevered member 36 (for ease ofmanufacture in accordance with the process described hereinafter) butmay be otherwise suitably attached. As previously discussed, the presentinvention does not require that the probe, which includes thecantilevered member 36, include the tip 40.

As previously discussed, for biological applications as well as use inviscous environments, it is important that a cantilever function well inliquids such as salt water. Undesirably, the hinge 12 of the cantilever10 of FIG. 1 warps when it becomes hydrated in a liquid and also warpswith temperature or contamination or other environmental conditions. Inorder to prevent such warping, in accordance with the present invention,the cantilevered member 36 is disposed intermediate the two arms 70 (andis shown to jut out beyond the free ends thereof), and the hinges 76 area pair of elongate or otherwise suitably shaped torsion bars or memberswhich torsionally interconnect the cantilevered member 36 to the arms 70respectively co-axially (as illustrated by the common longitudinal axis80 of the torsion bars 76 which defines the torsion bars 76 as beingco-axial) on opposite sides of the cantilevered member 36. Thus, thecantilevered member 36 is symmetrically and uniformly supported betweenthe arms 70 for rotational movement about the axis 80. The term “torsionbar”, as used herein and in the claims, is meant to refer to a memberwhich is twistable about an axis to allow movement of another member towhich it is connected about the axis. A hinge, for the purpose of thisspecification and the claims, is meant to include such a torsion bar.Such a symmetrical torsion bar arrangement is provided to allowsymmetrical control of movement of the cantilevered member 36 wherein itis free to twist or rotate in opposite rotational directions about thetorsion bar common axis 80 to allow the tip 40 to freely move upwardlyand downwardly. Each torsion bar 76 may have a length of, for example,about 10 microns. Its cross-sectional shape is rectangular, but may beotherwise suitably shaped. By “cantilevered member,” for the purposes ofthis specification and the claims, is meant a member which extendsbeyond its point(s) or means of support to act like a cantilever insupporting a tip or stylus or other surface engaging means for movementover a surface similarly as a stylus is supported in a record player.

In order to more easily and inexpensively make the sensor 38 (FIG. 3),including control of hinge thickness, as discussed more fullyhereinafter with respect to FIGS. 7 a to 7 s, the hinge 76 material isdifferent from the material of which the member 36 and tip 40 andstructure 14 are made. The member 36 and tip 40 and the supportingstructure 14 are made of silicon or other suitable material of athickness selected to be sufficient to provide the desired stiffness,while the hinge material is LPCVD (low pressure chemical vapordeposition) silicon nitride or other suitable material, such as, forexample, polyimide, gold, SU8, or plastic, whose thickness is selectedto be sufficiently thin to provide the desired compliancy (softness).Preferably, the thickness of the cantilevered member 36 as well as thesupport structure portion 72 (including arms 70) is the smallest that issuitably functional and has sufficient rigidity. In order to havesufficient rigidity, the thickness of each of the silicon cantileveredmember 36 and the silicon support structure portion 72 (including arms70) is at least about 1 micron, for example, about 5 microns (and ofcourse may be different if made of other materials). In order for eachof the silicon nitride hinges 76 to be sufficiently compliant, itscross-sectional height, illustrated at 77, is less than 400 nanometers,preferably less than about 100 nanometers, for example, about 50nanometers, to allow the probes to be softer than the softest presentlyavailable AFM cantilevers (about 0.01 N/m) and with resonances in waterabove 10 kHz (and of course may be different if made of othermaterials). Thus, cantilevered member 36 and support structure portion72 are on the order of 100 times thicker than the thickness 77 of thetorsion bars 76 so as to achieve the desired compliancy for the torsionbars 76 while providing the desired stiffness to the rest of thecantilever 38, and the hereinafter described cantilevers of FIGS. 5, 6,and 10 have similar thickness ratios. The torsion bar cross-sectionalwidth, illustrated at 79, is preferably between about 2 and 5 microns inorder to achieve the desired spring constant for the desired softness(compliancy), and hinge width scales linearly with the spring constant.

In accordance with the present invention, the compliant torsion bars 76are parts or portions or extensions of a thin layer or coating 78 ofmaterial which substantially spans (but need not span entirely) thelength and width of the sensor 38, i.e., it is applied (attached) to theback or bottom sides (opposite the side on which the tip is located) ofthe beam 36 and support structure 14 and extends therebetween to definethe compliant torsion bars 76. The torsion bars 76 are formed in afabrication process described hereinafter with respect to FIGS. 7 a to 7s. The thickness of the thin layer 78 of material is accordingly thesame as the height 77 of the torsion bars 76.

In order to make the outer surface of the portion of the layer 78 whichcovers the cantilevered member 36 (and similarly the outer surface ofthe portion of the layer 20 which covers member 16) suitably reflectivefor suitably reflecting the laser beam 48, it is coated with a suitablyreflective material, preferably gold, but other suitable reflectivematerials therefor include, but are not limited to, aluminum andsilicon. Because gold is much softer than silicon nitride (a preferredmaterial of which the torsion bars is made, as previously described), itwill desirably have a minimal effect on the spring constant.

Referring to FIG. 5, there is illustrated at 100 a sensor in accordancewith an alternative embodiment of the present invention. The sensor 100includes a first stiff support structure 102 and a second stiff supportstructure 104. The second support structure, similar to supportstructure 33, is a thinned extension of a handle chip 14. The firstsupport structure 102, which is similar to support structure 72, has apair of interconnected arms 106, similar to arms 70, which straddle astiff cantilevered member 108, similar to cantilevered member 36,supporting tip 110, similar to tip 40. Unlike the sensor 38 of FIGS. 3and 4, the cantilevered member 108 and torsion bars 112 of sensor 100are oriented laterally to the support structure 104, i.e., the arms 106straddle the cantilevered member 108 along the forward and rear sidesthereof (instead of along the lateral sides thereof). Moreover, the tip110 could be located anywhere on the cantilevered member 108 dependingon the desired sensitivity, for example, it is shown located generallycentrally on the cantilevered member 108. The arms 106 are connected tothe cantilevered member 106 by soft or flexible or compliant torsionbars 112 respectively, which are co-axial and otherwise similar totorsion bars 76 except that they are oriented to extend forwardly andrearwardly of the sensor 100 and are connected generally centrally ofthe cantilevered member 108.

A short compliant flexible thin hinge 114, similar to hinge 12, isprovided to connect the first and second support structures 102 and 104respectively. Unlike torsion bars, the flexible hinge 114 bends orflexes (does not twist, i.e., is not torsional) to allow movement of thefirst support structure 102 relative to the second support structure104. Similarly as described for FIGS. 1, 3, and 4, the hinge 114 andtorsion bars 112 are portions of a single layer, illustrated at 115, ofmaterial, similar to material layers 20 and 78, which covers and isattached to the bottomsides (but need not entirely cover) of the supportstructures 102 and 104 and cantilevered member 108, and the sensor 100may be constructed similarly as described with reference to FIGS. 7 a to7 s. Hinge 114 is provided for vertical sensitivity and compliance (zaxis 15 sensitivity), i.e., for sensitivity and compliance in verticalforce imaging as the tip 110 traverses a surface 32. Thus, hinge 114allows flexing movement (which is not torsional movement) of the firstsupport structure 102 relative to the second support structure 104. Thetorsion bars 112 are provided for lateral or twisting sensitivity andcompliance (x axis 17 sensitivity), i.e., for sensitivity and compliancein lateral force imaging as the tip 110 traverses a surface 32. Thus,the sensor 100 of FIG. 5 is provided to desirably achieve enhancedlateral and vertical force imaging.

Referring to FIG. 6, there is illustrated at 120 a two-axis sensor(i.e., allowing rotational movement about two torsion bar axes) inaccordance with another embodiment of the present invention. The sensor120 includes first and second rigid support structures 122 and 124 and arigid cantilevered member 126 having a tip 128, all similar to thecorresponding support structures 72 and member 36 and tip 40 except asotherwise shown or hereinafter described. The first support structure122, similar to support structure 33, is a thinned extension of a handlechip 14. The second support structure 124 is shaped to define a firstpair of generally spaced parallel arms 130 and 132 which are connectedat ends thereof by a second arm 134. The first arms 130 and 132 straddlethe cantilevered member 126, and the first arm 130 extends beyond thecantilevered member 126 as well as beyond the end of the other first arm132. Another second arm 136 extends from the other end of first arm 130in a direction generally toward the longitudinal axis of the other firstarm 132 and generally parallel to the other second arm 134. The firstsupport structure has a pair of spaced generally parallel arms 138 and140 which straddle the second support structure 124, its arms 138 and140 extending generally parallel to arms 134 and 136. A first pair offlexible elongate torsion bars 142 co-axially (along longitudinally axis144) connects (extending forwardly and rearwardly of the sensor) thecantilevered member 126 to the first arms 130 and 132. A second pair offlexible elongate torsion bars 146 co-axially (along longitudinally axis148) connect (extending laterally of the sensor) the first supportstructure arms 138 and 140 to the second arms 134 and 136 respectively.The torsion bars 142 and 146 are similar to torsion bars 76. Similarlyas described for FIGS. 3 and 4, the torsion bars 142 and 146 areportions of a single layer, illustrated at 147, of material, similar tomaterial layer 78, which covers and is attached to the bottomsides (butneed not entirely cover) of the support structures 122 and 124 andcantilevered member 126, and the sensor 120 may be constructed similarlyas described with reference to FIGS. 7 a to 7 s.

An elongate arm 150 extends from the free end of arm 132 outwardly(forwardly) or away from cantilevered member 126 in the axial direction144. A second tip 156, similar to tip 40, is disposed on the outer endportion of arm 150. Thus, the second tip 156, which is on the secondsupport structure or outer gimbal 124, is disposed at some distance fromthe tip 128, which may be called the “sample tip.” It should be notedthat torsion bars 142 allow twisting movement of the cantilevered memberor inner gimbal 126 about axis 144, that torsion bars 146 allow twistingmovement of the second support structure or outer gimbal 124 (as well asinner gimbal 126) about axis 148, and that axes 144 and 148 aregenerally normal or orthogonal to each other. The support structures 122and 124 as well as cantilevered member 126 may be, for example, 10microns thick.

What is measured by an AFM is traditionally the force on the sample tip128 of the compliant cantilever. Since the AFM is in the dimensionalrange on the order of 10 cm., any movement of any part of the AFM orstage will undesirably deflect the sample tip 128 while it is otherwisebeing deflected by the object's surface 32, and the tip 156 willsimilarly be deflected. Also, building vibrations and the like can shake(deflect) the cantilever relative to the object 34 being investigated.Such unwanted deflections create low frequency coherent noise (includingnoise caused by microscope drift and environmental noise). It isconsidered desirable that such unwanted coherent noise be removed toincrease measurement accuracy and precision increase while providing theability to conduct longer experiments. Its removal is also desired toremove drift in the z axis 15 direction as a significant factor andthereby reduce demands on microscope design. In order to remove suchunwanted noise, in accordance with the present invention, the tip 156 isused as a reference tip. Since the two axes 144 and 148 are orthogonal,they encode the two positions independently from the same laser beam 48whereby the need for two separate lasers is eliminated. Twistingmovement about axis 144 is picked up only by sample tip 128 to provideinformation about the surface 32. One axis of the photodetector 52records the reference position, and the other axis thereof records thesample position. Note that z-axis 15 movement is however picked up byboth tips 128 and 156. Unwanted noise is removed by taking thedifference in the movement using the output from the photodetectors,using principles commonly known to those of ordinary skill in the art towhich this invention pertains. Thus, all common mode noise may becanceled.

The sensor 120 may also be used for direct measure of sample stiffness.In order to do so, The two axes (torsion bars 142 and 146) are providedwith different stiffnesses (to provide a differential spring constantfor the sample), using principles commonly known to those of ordinaryskill in the art to which this invention pertains. Cantilever deflectionis a linear combination of the sample compliance and the cantilevercompliance. Thus, since the two axes have different stiffnesses, asingle measurement is made which utilizes deflections from the two axesto solve algebraically for the sample stiffness directly.

The position and shape of the reference tip 156 can be varied. If thereference tip 156 is placed close to the sample tip 128, a highresolution differential contrast image of topology is developed. If thereference tip 156 is placed at the end of an extended arm (i.e., arm 150having a length, illustrated at 154, of, for example, about 100microns), the sample tip 128 can then be placed, for example, on a celland the reference tip 156 placed, for example, on a coverslip to allowthe sample tip sufficient clearance above the cell. For increased detailresolution, the two tips 128 and 156 can be placed closer together (forexample, an arm length 154 of about 5 microns) to create a differentialcontrast image of topology or compliance. For single molecule forcespectroscopy, the dual axis sensor may provide not only improved highfrequency response but also improved low frequency response by removingdrift. If the reference tip 156 is large (perhaps flat), it will averageover a rough substrate.

When made of thin silicon nitride to be soft, the reference torsion bars146 maximize force sensitivity. Alternatively, the reference torsionbars 146 may be made of silicon (with the layer 147 of silicon nitridecovering the bottomsides thereof in accordance with the process of thepresent invention) to be stiff to create a more stable reference contactwith maximum frequency response.

Referring to FIGS. 10 and 12, there is illustrated at 220 a two-axissensor (i.e., having two independent orthogonal data axes 244 and 248for rotational movement thereabout, as illustrated at 245 and 249respectively, and having a tip 228 which is asymmetric to both axes 244and 248, in accordance with another embodiment of the present invention.The sensor 220 is shown in FIG. 12 applied to anchor point 252 detectionof a polymer, illustrated at 250, such as a DNA molecule, a protein,etc. tethered to the tip 228, and may have various other applications,as discussed hereinafter.

The sensors shown in FIGS. 13 to 17 also are two-axis sensors. However,these oscillators (FIGS. 10, 11, and 13 to 17) may be used for otherpurposes other than as sensors. For example, they may be used astwo-axis accelerometers. In addition, a tip is not required for areference probe in these sensors. For example, the end of the referencelever may be permitted to just slide over the surface under examination.

The sensor 220 includes first and second rigid support structures 222and 224 and a rigid cantilevered member 226 having tip 228, all similarto the corresponding support structures 72 and member 36 and tip 40except as otherwise shown or hereinafter described. The first supportstructure 222, similar to support structure 33, is a thinned extensionof a handle chip 14. The second support structure 224 is shaped todefine a first pair of generally spaced parallel arms 230 and 232 whichare connected at ends thereof by a second arm 234. The first arms 230and 232 straddle the cantilevered member 226, and the first arm 230extends beyond the cantilevered member 226 as well as beyond the end ofthe other first arm 232. Another second arm 236 extends from the otherend of first arm 230 in a direction generally toward the longitudinalaxis of the other first arm 232 and generally parallel to the othersecond arm 234. The first support structure 222 has a pair of spacedgenerally parallel arms 238 and 240 which straddle the second supportstructure 224, its arms 238 and 240 extending generally parallel to arms234 and 236. A first pair of flexible elongate torsion bars 242co-axially (along longitudinally axis 244) connect (extending forwardlyand rearwardly of the sensor) the cantilevered member 226 to the firstarms 230 and 232. A second pair of flexible elongate torsion bars 246co-axially (along longitudinally axis 248) connect (extending laterallyof the sensor) the first support structure arms 238 and 240 to thesecond arms 234 and 236 respectively. The torsion bars 242 and 246 aresimilar to torsion bars 76. Similarly as described for FIGS. 3 and 4,the torsion bars 242 and 246 are portions of a single layer, illustratedat 247, of material, similar to material layer 78, which covers and isattached to the bottomsides (but need not entirely cover) of the supportstructures 222 and 224 and cantilevered member 226, and the sensor 220(as well as the sensors hereinafter described with reference to FIGS. 13to 17) may be constructed similarly as described with reference to FIGS.7 a to 7 s and attached to chip 14 similarly as shown in FIG. 10.

The optical lever 54 (a laser beam deflecting from the central pad 226)is used to detect deflection of the central pad 226. Each of thedeflection axes 244 and 248 will have a well-defined and small springconstant defined and calibrated based on the geometric and materialproperties, using principles commonly known to those of ordinary skillin the art to which this invention pertains. This provides a desirablyvery compliant relation between the force applied and the levermovement. The use of two deflection axes 244 and 248 provides theability to reduce the spring constants for improved sensitivity.

The cantilevered member 226 has a portion 227 which off-sets the tip 228from both of the torsion bar axes 244 and 248 (i.e., the tip 228 is notlocated on either of the axes 244 or 248 so that it is asymmetricthereto) to, in one application, allow thermal noise to be filtered(thermal noise reduction) as follows. With only one axis, one cannotdetect whether a detection is noise or a signal. Since random noise willproduce deflection of the pad 226 that can be detected bi-laterally(deflects the pad 226 along both axes 244 and 248 independently), noiseis cross-referenced in two channels and filtered out (the two responsesare averaged), improving noise sensitivity by the square root of 2. Thenormal laser beam 48 illuminates the mirror (i.e., the gold-covered backof the silicon pad 226), and the two orthogonal torsion bar axes 244 and248 can be decoded by the photodetector 52 in 4 quadrants (bottom-top,left-right), which measures movement of the laser beam 54, which isrelated to the movement of the tip 228. Z axis noise at higherfrequencies is removed by taking the difference of the two signals,after suitable scaling for differences in optical gain of the two axes244 and 248, and intensity fluctuations of the laser system may also besuppressed. Each of the vibration modes shows up in the orthogonaldirection on the photodetector 52. Flexing of the outer gimbal orsupport structure 224 shows up at, for example, 26 kHz on the bottom-topphotodetector channel, while the inner pad or cantilevered member 226has, for example, a 95 kHz resonant peak on the left-right channel.

This probe 220 may alternatively be used for simultaneous friction andtopography when tip 228 is placed coaxially with rotational axis 248 andis displaced from axis 244. In order to use the probe 220 for scanning,z-axis movement is encoded by rotation about axis 244 while samplefriction, utilizing the length of tip 228 as a lever, rotates the mirror(cantilevered member) 226 about axis 248.

Another application of two-axis probe 220 is for resolution of the angle(force manipulation) between a tip-linked polymer 250 and the AFM Z axis15, a situation typical in dynamic force spectroscopy. Off-normal anglescause underestimation of sample stiffness since only the Z component ismeasured. With high compliance torsion bars 242 and 246 and at smallangles, the deflection angle of the cantilevered member 226 will tend tobecome normal to the axis of the polymer 250. By using x-axis and y-axisfeedback to minimize the tipping angle, the center of attachment to thetension axis can be made normal to the substrate. The object is toobtain a vectorial representation of where the anchor point 252 is. Thisis achieved by measuring the force on both axes 244 and 248 and addingvectorially or alternatively by moving the tip 228 to a position suchthat there is zero force on one axis and then measuring the force on theother axis, as discussed in greater detail hereinafter.

The lever 226 senses force on the tip 228 (located distance L from thecenter of the axes 244 and 248) from two orthogonal directions, andforce applied to the tip 228 results in a well-defined deflection of theoptical lever 54 (PDT top-PDT bottom ˜L*sin a, and PDT left-PDT right˜L*cos a, where PDT is the photodetector measurement of movement involts). This directional resolution may be used to detect origin offorce applied to the tip 228. Thus, the cantilever tilts to be normal tothe axis of the tether 250 to provide a precise measure of sample 250stillness. Precision can be further improved by using the tilt angleinformation to feed back to the xy scanner to position the tip 228directly above the attachment point 252 of the tether 250. Prior to thebeginning of an experiment, the angular response or optical sensitivityof the lever is calibrated by monitoring deflection due to near-normalforce applied to the tip 228 (force-distance curve on glass). Then, withthe tether 250 attached, a comparison of the force applied to the tip228 that is off-normal the deflection response of the lever to thecontrol response will reveal force directionality. In this manner, theanchor point 252 of the tether 250 may be located in space by minimizingthe difference between the control and experimental responses.

Except as described otherwise herein, the sensors illustrated in FIGS.13 to 17 are two-axis sensors which are similar to sensor 220 of FIGS.10 and 12, the primary differences being the locations of the torsionbar axes and the tip(s).

Referring to FIG. 13, there is shown generally at 300 a sensor which maybe used for highly sensitive friction experiments. Sensor 300 includesfirst and second rigid support structures 302 and 304 respectivelyconnected by a pair of torsion bars 306 providing torsion axis 308 forrotational movement of the second support structure (cantileveredmember) 304 thereabout, the first support structure 302 being connectedto the chip 14. A cantilevered member or pad 310 is disposed between apair of arms 316 and 318 of second support structure 304 and connectedthereto by a second pair of torsion bars 312 respectively providingtorsion axis 314 for rotational movement of the cantilevered member 310thereabout. The leg 318 terminates with its connection to its respectivetorsion bar 312. The x and y axes 308 and 314 are perpendicular orotherwise orthogonal to each other, as are the axes for other sensorsdisclosed in this specification.

Cantilevered member 310 has an L-shaped extension comprising a leg 320which extends in the y-direction 19 beyond the leg 318 and another leg322 which extends from the outer end of leg 320 in an x-direction 17 tosupport a tip 324 on or along the y axis 314 so that it is sensitive tothe torque produced by dragging the tip 324 across a surface 32. As aresult, it is provided so as to be optimal for frictional measurementsdue to a large increase in optical gain occasioned thereby. The x axis308 is used to detect sample topology (force contact or force setpoint). Such a lever is provided to allow ultra-sensitive frictionalrecording (PDT left-PDT right) at highly controlled contact forces (PDTtop-PDT bottom). The tip 324 is spaced from the x axis 308 and from thecenter of the sensor 300. However, the position of the tip 324 can bevaried along the length of the y-axis 314 for additional recording orfeedback applications.

Referring to FIG. 14, there is shown generally at 400 a sensor which mayalso be used for friction experiments, more particularly to examinefrictional characteristics of biological cells. Sensor 400 includesfirst and second rigid support structures 402 and 404 respectivelyconnected by a pair of torsion bars 406 providing torsion axis 408 forrotational movement of the rectangular second support structure(cantilevered member) 404 thereabout, the first support structure 402being connected to the chip 14. A cantilevered member or pad 410 isdisposed within the rectangular second support structure 404 andconnected thereto by a second pair of torsion bars 412 providing torsionaxis 414 for rotational movement of the cantilevered member 410thereabout. The x and y axes 408 and 414 are perpendicular or otherwiseorthogonal to each other, as are the axes for other sensors disclosed inthis specification.

Unlike other sensors shown in this specification, cantilevered member410 does not have a tip. Instead, biological cells are attached orcultured on the cantilevered member 410 and then brought into contactwith the sample at controlled interaction forces to examine frictionalcharacteristics of the biological cells in two orthogonal directionslooking for anisotropy of the sample.

Referring to FIGS. 15 and 16, there is shown generally at 500 a sensorwhich may be used for drift-free and reduced noise operation. Sensor 500includes first and second rigid support structures 502 and 504respectively connected by a pair of torsion bars 506 providing torsionaxis 508 for rotational movement of the rectangular second supportstructure (cantilevered member) 504 thereabout, the first supportstructure 502 being connected to the chip 14. A cantilevered member orpad 510 is disposed within the rectangular second support structure 504and connected thereto by a second pair of torsion bars 512 providingtorsion axis 514 for rotational movement of the cantilevered member 510thereabout. The x and y axes 508 and 514 are perpendicular or otherwiseorthogonal to each other, as are the axes for other sensors disclosed inthis specification.

A leg 516 extends in the y direction 514 outwardly from the second rigidsupport structure 504 (away from the chip 14) and supports a first tip518 on or along the y axis 514, which tip 518 is accordingly offset fromthe x axis 508. The cantilevered member 510 supports a second tip 520 onor along the x axis 508 and offset from the y axis 514.

Tip 518 and the rotation about the x axis, as indicated by 522, is usedfor referencing (measuring the position on the substrate 34). Tip 520and the rotation about the y axis, as indicated by 524, is used formeasurement of the position of the sample (the actual physical force ofattachment). The reference tip 518 is placed to touch the substrate 34first and records the sum total of all deflections of the microscoperelative to the substrate 34 (PDT top-PDT bottom). The measurement orsample tip 520 is caused to engage and interact with the sample understudy and record PDT left-PDT right to monitor the experiment forces(with option to cross-reference with the reference signal). The computer56 then scales and subtracts the two measurements to remove all commonmode noise.

Referring to FIG. 17, there is shown generally at 700 a sensor which maybe used for measurement of friction while referencing the substrate 34(optimize lateral recording). Sensor 700 has first and second rigidsupport structures 702 and 704 respectively connected by a pair oftorsion bars 706 providing torsion axis 708 for rotational movement ofthe rectangular second support structure (cantilevered member) 704thereabout, the first support structure 702 being connected to the chip14, similarly as the corresponding elements in sensor 500. Acantilevered member or pad 710 is disposed within the rectangular secondsupport structure 704 and connected thereto by a second pair of torsionbars 712 providing torsion axis 714 for rotational movement of thecantilevered member 710 thereabout, similarly as the correspondingelements in sensor 500. The x and y axes 708 and 714 are perpendicularor otherwise orthogonal to each other, similarly as the correspondingelements in sensor 500. A leg 716 extends in the y direction 714outwardly from the second rigid support structure 704 (away from thechip 14) and supports a reference tip 718 on or along the y axis 714,which tip 718 is accordingly offset from the x axis 708, similarly asthe corresponding elements in sensor 500.

The cantilevered member 710 supports a friction measurement tip 720 onor along the x axis 708 and also on or along the y axis 714 (i.e., atthe intersection of the x and y axes) to optimize measurement offriction while also referencing the substrate 14. The reference tip 718is placed onto the substrate 14 and used to monitor position of thesubstrate 14 and noise. When the sample is sheared across (either x or ydirections), the tip 720 is optimally sensitive but has little or nosensitivity to normal force (z direction 15) due to its being at theintersection of the axes 708 and 714.

Reference torsion bars 706 may be made soft using thin silicon nitrideto provide higher precision, as otherwise discussed herein particularlywith reference to FIGS. 7 a to 7 s, or made stiff using thick silicon toreduce noise by strongly coupling to the substrate 14.

Referring to FIGS. 7 a to 7 s, there is illustrated a sequence of stepsfor mass production of sensor 10 (FIG. 7 s). However, one of ordinaryskill in the art can use similar principles and apply similar steps toproduce sensors 38, 100, and 120 and the other sensors shown herein andother oscillators with torsion bars with a wide variety of geometriesand mechanical properties (sizes, spring constants, etc.), includingcustom shapes and modes of motion, on a single inexpensive die usingstandard micro-fabrication equipment. For example, FIG. 7 s could beconsidered to partially illustrate sensor 38 wherein reference numeral12 instead illustrates one of the torsion bars 76, reference numeral 16instead illustrates cantilevered member 36, reference numeral 18 insteadillustrates tip 40, and reference numeral 33 instead illustrates thinsupport structure 70. Thus, the principles discussed hereinafter forformation of the hinge 12, cantilevered lever 16, tip 18, and thinsupport structure 33 should be considered as also applying, asapplicable, to the formation of each of the torsion bars 76,cantilevered lever 36, tip 40, and thin support structure 70respectively.

It is considered desirable that the cantilevered member 12 as well asthe support structure be of adequate stiffness so that the entire sensordoes not undesirably flex while it is also considered desirable that thetorsion bars or hinges have the flexibility or softness for the desiredcompliance, so that they can suitably act as springs in allowing the tipto move freely over the surface 32 (or otherwise detect the surface 32)similarly as a stylus is allowed to move freely over a record beingplayed. However, when the sensors are made entirely of silicon, thetorsion bars or hinges cannot be easily made thin enough, unlessexpensive E-beam lithography is used, to be sufficiently compliant,because it is difficult to keep the etchant from continuing to eat awayand “dissolve” the hinge when the desired thickness (i.e., very thin) isobtained. On the other hand, when the cantilevers are made entirely ofsilicon nitride, the cantilevered members and support structure areconsidered to not be sufficiently stiff. In order to obtain the desiredstiffness for the cantilevered members and support structure whileproviding the desired flexibility or softness to the torsion bars orhinges so that they suitably act as springs, without the use ofexpensive E-beam lithography, in accordance with the present invention,the cantilevered members 16 (and tips 18) and support structure 14 aremade of one material and the hinges 12 (or 76) are made of anothermaterial which is applied as a thin layer 20 to the first material andwhich is desirably insensitive to or at least substantially lesssensitive to the etchant used for etching the first material. Theetchant is thus selective for the first material. By an etchant being“selective” for one of two materials is meant, for the purposes of thisspecification and the claims, that the etchant is such as to readilyetch the one material as compared to the other material, which is leftsubstantially unetched, during an etching process. The first material isthen etched all the way through it to the layer of second material(wherein the etching ceases due to the insensitivity of the secondmaterial to the etchant) to form the thin compliant hinge 12. Theetchant is

In order to mass produce such composite material micro-mechanicaloscillators such as oscillator 10 (wherein the cantilevered member 16thereof oscillates), in accordance with the present invention, aquantity of sites on the wafer 27 (FIG. 11) of a first material such assilicon are etched to define support structures (handle chips 14) asmore particularly described hereinafter, the layer 20 of the secondmaterial is deposited on each site (over the entire wafer 27), and theoscillator hinges 12 (or torsion bars 76) are patterned in the sites,including etching all the way through the first material to form eachhinge of the second material, as more particularly describedhereinafter. As used herein and in the claims and unless otherwisespecified or apparent in this specification, the term “flexible hinge”is meant to include “torsion bar.” Each handle chip 14 is made to beeasily removed from the silicon wafer 27 by etching all the way throughthe wafer to define three handle chip sides, as illustrated at 29 inFIG. 11, and pre-etching along the fourth side, then, when the chip hasbeen formed, breaking along the resulting pre-etched edge 25 to removeit from the wafer 27. The resulting oscillator 10 is thus supported on alarge silicon die (handle chip 14) which can advantageously be used foreasy manipulation and device support and can be produced of standardmicro-fabrication materials (discussed hereinafter) to have very thincompliant hinges or torsion bars and very stiff support structures andcantilevered members.

The silicon wafer 27 is preferably of the type having a 100-orientation(<100>), a crystal orientation wherein the wafer is formed by cuttingalong a plane known as the 100-oriented plane. Silicon wafers withdifferent crystal orientations (such as, for example, <110>) may beused, but subsequent processing thereof, as described hereinafter, willresult in different die (handle chip) shapes.

As shown in FIG. 7 b, the topside 208 and the bottomside or backside 202of the wafer 200 are simultaneously coated with layers 216 and 217respectively of silicon nitride (preferably high quality silicon-richSiN) using the well known in the art technique of low pressure chemicalvapor deposition (LPCVD). These silicon nitride layers 216 and 217 willlater be removed (they are shown to have been removed in FIG. 7 g) afterbottomside silicon nitride layer 217 serves as a KOH etch mask. Althoughthe LPCVD technique normally results in both layers 216 and 217 beingdeposited on the wafer, it should be noted that the present inventiondoes not require that layer 216 be applied to the wafer.

A layer 218 of photoresist is then applied to the bottomside siliconnitride layer 217, as shown in FIG. 7 c. A portion of the photoresistlayer 218 is removed, as seen in FIG. 7 d, using UV (ultraviolet light)contact-microlithography and aqueous development, which are well knowntechniques to those of ordinary skill in the art to which this inventionpertains, to expose a portion 219 of the bottomside silicon nitridelayer 217 for the hereinafter described etching (the unremoved portion221 of the photoresist layer 218 serving to mask the portion of thelayer 217 not to be etched).

Referring to FIG. 7 e, with the photoresist portion 221 serving as amask, the silicon nitride portion 219 is removed by the well known inthe art technique of reactive ion etching (RIE) with CHF3 (trifluromethane), leaving silicon nitride portion 223. The photoresist layer221, no longer needed, is then removed.

As seen in FIG. 7 f, with silicon nitride portion 223 serving as an etchmask, the silicon wafer 27 is etched using aqueous potassium hydroxide(KOH), and the etching proceeds anisotropically, in accordance withprinciples commonly known to those of ordinary skill in the art to whichthis invention pertains, to define a crater 204 having inwardly slantedwalls 206 and 207, and slanted wall 206 will become coated, ashereinafter described, to define the slanted wall 19 of the handle chip14, as best seen in FIG. 7 s. The pattern is defined by opticalphoto-lithography and RIE (reactive ion etch) etch of LPCVD siliconnitride. The etch is conducted to proceed most of the way through thethickness of the wafer 27, leaving centrally along the topside 208 ofthe wafer 200 a thin membrane 210 (having a thickness, illustrated at402, in the neighborhood of 10 to 20 microns, for example, about 10microns) that is stiff in comparison with the much thinner to be formedhinge 12. This membrane 210, which will be thinned more (but still besufficiently thick to have the desired stiffness) during furtherprocessing as will be hereinafter described, will later be formed intothe cantilevered member 16, tip 18, and the support structure front endportion 33 of the sensor 10, as best seen in FIG. 7 s.

In addition to providing a chip handle (enlarged portion) with a slopedfront end 19 to increase the optical clearance, the anisotropy of theKOH etch is also used to produce chips (sensors) that are solidlysupported by the wafer throughout processing and can be easily removedfrom the wafer by controlled fracture along pre-etched edge 25 (FIG. 9).

A common problem for KOH-assisted definition of silicon blocks is convexcorner undercutting. During the etch, low density high etch-rate Millerplanes are exposed resulting in significant rounding. See W. Chang Chienet al, “On the Miller-indices Determination of Si-100 Convex CornerUndercut Planes,” Journal of Micromechanics and microengineering, vol.15, 2005, pp 833-842; and X. Wu et al, “Compensating Corner Undercuttingin Anistropic Etching of (100) Silicon,” Sensors and Actuators, vol. 18,1989, pp 207-215. A set of compensation structures (which may be calleddog-ears, illustrated at 283 in FIG. 8, and which are well known in theart to which the present invention pertains), which are empiricallydesigned to protectively minimize undercut, are preferably provided atthe corners to allow production of nearly rectangular chip handles sizedto fit standard AFM holders (1.8 mm×3 mm). They can be empiricallydesigned and provided using principles commonly known to those ofordinary skill in the art to which this invention pertains, followingthe procedures described in the above W. Chang Chien et al and X. Wu etal articles.

To fabricate the membrane 210 desirably of controlled thickness,inexpensive and reproducible in situ rulers are preferably used, whichcan be used in accordance with principles commonly known to those ofordinary skill in the art to which this invention pertains using theprocess as described in P. Chang, “A method Using V-grooves to Monitorthe Thickness of Silicon Membrane with uM Resolution,” J. Micromech.Microeng., vol. 8, 1998, pp 182-187. For example, the membrane 210 maybe initially produced to have a thickness, illustrated at 402 in FIG. 7g, of 10 to 20 microns, then thinned during tip production to have athickness, illustrated at 404 in FIG. 7 k, of 5 to 10 microns, thenfurther thinned during the tip making process to a thickness,illustrated at 406 in FIG. 7 s, of about 5 microns (the resultingthickness of the cantilevered member 16 and thin support structureportion 33).

In addition to being of controlled thickness, the bottomside 212 (FIG. 7s) of the membrane 210 (the side which will be opposite the tip) shouldbe optically flat to suitably reduce light scattering as the laser beam48 is reflected from the cantilevered member surface. In order toproduce such an optically flat membrane 210, in accordance with apreferred embodiment of the present invention, the procedure which isused is one in which KOH and IPA (isopropyl alcohol) are used asfollows. Approximately an hour before the finish of the KOH etch toproduce the membrane 210, the bath temperature is decreased to about 50to 60 degrees C., and about 10 to 15 percent IPA is added to the 50% KOHsolution. The etch then proceeds at a lower rate, resulting in asmoother surface.

As illustrated in FIGS. 7 g and 7 h, topside layer 216 and bottomsidelayer 217 of silicon nitride are removed and replaced by a thin filmstack of (1) first a layer 214 and a layer 215 (for the topside andbottomside respectively) of silicon dioxide then (2) secondly a layer263 and a layer 265 (for the topside and bottomside respectively) ofsilicon nitride (preferably high quality silicon-rich SiN) using thewell known in the art technique of low pressure chemical vapordeposition (LPCVD).

The bottomside SiN layer 265 is later patterned into the hinge 12, asbest seen in FIG. 7 s. In order to achieve a suitable hinge compliancy,the silicon nitride layer 265 has a thickness less than about 400nanometers, preferably less than about 100 nanometers, for example,about 50 nanometers. It may be as thin as about 10 nanometers.

The topside silicon dioxide layer 235 is provided to serve as protection(i.e., support the tip region) until the final release, i.e., it isshown in FIG. 7 r and is shown as having been removed in FIG. 7 s. Asilicon dioxide thickness of about 100 nanometers is considered to beadequate to protect the hinge during processing. The topside siliconnitride layer 263 is provided so that portion 231 (FIGS. 7 j and 7 k)thereof serves as a mask for thinning the membrane 210 and formation ofthe tip 18.

The bottomside thin film layers 215 and 265 bear intrinsic mechanicalstress, tensile for silicon nitride and compressive for silicon dioxide,which could cause hinge warping (when the hinge is asymmetric). However,when the hinge means is two symmetric torsion bars 76 (as in FIG. 3),such stress-induced curvature is advantageously not observed to occur.

The production of the tip 18 begins with the application of a layer 225of photoresist onto the topside silicon nitride layer 263, as seen inFIG. 7 i, followed by micro-lithographic removal of the photoresist 225except for a portion 227 thereof for masking the position in the waferwhere the tip 18 will be formed, as seen in FIG. 7 j. With the exceptionof portions 231 and 233 beneath the photoresist mask 227, the topsidelayers 263 and 214 respectively are patterned (removed) by RIE etchingdown to the thin membrane 210, leaving layer stack portion 227, 231, and233, as shown in FIG. 7 j. The photoresist mask 227 (no longer needed)is then removed.

As illustrated in FIG. 7 k, the top surface to a depth of, for example,5 microns, of the membrane 210 next to and on both sides of the stackportions 231 and 233 is removed by a short reactive ion etch (RIE),forming a masked silicon island or raised portion (for example, 5 squaremicrons) which will become the tip 18. The tip 18 is formed by acontrolled undercutting of the small masked island, as illustrated at224, using any of several well known in the art processes, which aredescribed in A. Boisen et al, “AFM Probes with Directly FabricatedTips,” Journal of Micromechanics and Microengineering, vol. 6, 1996, pp58-62; T. Albrecht et al, “Microfabrication of Cantilever Styli for theAtomic Force Microscope,” Journal of Vacuum Science & Technology A, vol.8, 1990, pp 3386-3396; J. Brugger et al, “Silicon Cantilevers and Tipsfor Scanning Force Microscopy,” Sensors and Actuators A: Physical, vol.34, 1992, pp 193-200; and J. Itoh et al, “Fabrication of an Ultrasharpand High-aspect-ratio Microprobe with a Silicon-on-insulator Wafer forScanning Force Microscopy,” J. Vac. Sci. Technol. B, vol. 13, 1995, pp331-333. The tip 18 is formed when the etchant undercuts the island(layer stack 231 and 233) sufficiently to lift the island off (whichlayer stack 231 and 233 is discarded as no longer needed), as seen inFIG. 71. As used herein and in the claims, the term “undercutting” isdefined as the cutting or etching of a wafer portion which is beneath amasking material. We have produced high-aspect (for example, a base of 5microns and a height of 5 microns) atomically sharp tips by undercuttingLPCVD silicon dioxide (stack portion 233) with SF₆ (sulfur hexafluoride)while silicon nitride stack portion 231 serves to protect the tip regionduring such processing. Since three types of masks (silicon nitride,silicon dioxide, and photoresist) are available, a variety of otherchemistries may be used to produce the tip 18. Silicon nitride (SiN) isan excellent mask for KOH etch; silicon dioxide is a good mask for deepreactive ion etch (DRIE); and photoresist is a good mask for SF₆. Wehave also made lower-aspect-ratio (height to width of, for example,about 1.2) tips, with cone angles of about 70 degrees, using siliconnitride (SiN) as a mask and KOH for an anisotropic undercut. Such tips,which may have broad bases, four-fold symmetry, and atomically flatwalls, have well-defined shapes and are considered suitable for use withsoft biological materials. We have been able to produce taller tips bypre-etching a small post using DRIE prior to release by SF₆ and KOH. Seethe previously cited Brugger et al, Boisen et al, and Albrecht et alarticles. Tips may be further sharpened by oxidation without additionalprocessing if dry silicon dioxide is grown during the further processingsteps discussed hereinafter. See R. Marcus et al, “The Oxidation ofShaped Silicon Surfaces,” Journal of the Electrochemical Society, vol.129, 1982, pp 1278-1282; and R. Marcus et al, “Formation of Silicon Tipswith Less-than-1 Nm Radius,” Applied Physics Letters, vol. 56, 1990, pp236-238. Once the tip 18 is produced, it is protected throughout therest of the process by applying over the wafer topside a thin layer 235of plasma-enhanced chemical vapor deposited (PECVD) silicon dioxide, asillustrated in FIG. 7 m.

After forming the tip 18, the hinge 12 and oscillator (cantileveredmember) 16 are defined or formed, beginning with deposition ofplasma-enhanced chemical vapor deposited (PECVD) silicon dioxide aslayer 235 and then the application of a layer 237 of photoresist, asshown in FIG. 7 m, wherein the layers 235 and 237 serve as a doublemask. A portion of the photoresist layer 237 above where the hinge 16will be, illustrated at 267 in FIG. 7 m, is removed by imaging the hinge(or torsion bar) shape therein. With the photoresist layer 237 maskingthe rest of the silicon dioxide layer 235, the hinge shape, illustratedat 241, is then etched in the silicon dioxide layer 235 using CHF3 RIE,as shown in FIG. 7 n. The photoresist mask 237 (no longer needed) isthen removed.

As illustrated in FIG. 7 o, a new photoresist layer 239 is applied(spun-on) over the patterned hinge shape 241 and over the silicondioxide layer 235. Next, the entire probe structure (handle chip) 14 isimaged onto the photoresist 239, resulting in removal of a portion ofthe photoresist mask 239, as shown at 243, which corresponds to thesilicon etching illustrated at 29 in FIG. 11 along three of the sides ofthe handle chip 14. With the photoresist layer 239 masking the rest ofthe silicon dioxide layer 235, The handle chip shape is then etched intothe silicon dioxide layer 235, as illustrated at 245 in FIG. 7 p, usingCHF3 RIE.

Referring to FIG. 7 q, the silicon around the cantilevered member 16(with the exception of the area where the hinge 12 will be located) andaround the three sides of the handle chip, as illustrated at 29 in FIG.11, is then etched, as shown at 247, using the inductively coupledplasma reactive ion etch (ICP-RIE) C₄H₈/SF₆, as described in F. Laermeret al, “Challenges, Developments and Applications of Silicon DeepReactive Ion Etching,” Microelectronic Engineering, vol. 67-68, 2003, pp349-355, which procedure can be followed using principles commonly knownto those of ordinary skill in the art to which this invention pertains.Then CHFR-RIE etching is used to remove the underlying silicon dioxideand silicon nitride layer portions, as shown at 251 and 253respectively.

As seen in FIG. 7 r, the photoresist mask 239 is then removed, exposingthe silicon dioxide layer 235 which was previously patterned with thehinge pattern 241. In accordance with the present invention, an ICP-RIE(inductively coupled plasma reactive ion etch) etch is applied inaccordance with principles commonly known to those of ordinary skill inthe art to which this invention pertains, as illustrated at 255, all theway through the silicon 200 to expose the silicon dioxide and siliconnitride portions defining the hinge 12 thereby forming the hinge 12.Since the etchant is much more selective for silicon than for silicondioxide, the etching substantially stops when the silicon dioxide layer215 is reached. Other suitable etchants, such as, for example, KOH,having similar selectivity can alternatively be used. Such an etchantwith the desired selectivity is thus applied to a silicon (or othersuitable material) wafer having a layer of silicon nitride (or othersuitable material) so as to etch entirely through the wafer to exposeand therefore form the oscillator hinge easily and inexpensively(without the need for expensive e-beam lithography) and precisely to thedesired hinge thinness (which is the thinness of the layer 265 asapplied) for the desired hinge compliancy.

Finally, with reference to FIG. 7 s, in order that the hinge 12 maydesirably be further softened (compliancy increased), the hinge patternportion of the silicon dioxide layer 215, optionally, is selectivelyremoved (i.e., without etching into the silicon nitride layer 265), asshown at 257, using 10-50% aqueous HF (hydrogen fluoride), morespecifically described hereinafter, to further release the siliconnitride hinge 12. The silicon dioxide layer 235 is similarly removed touncover the tip 18 (which until this step has been protected by thesilicon dioxide layer 235). Since etch selectivity for silicon nitrideover silicon dioxide increases with decreasing HF concentrations, wehave used an aqueous mixture of 20% HF in 40% ethanol and 40% water, theethanol being added to the mixture to reduce surface tension. At thisconcentration of HF, the selectivity of silicon dioxide to siliconnitride is on the order of 100 to 1, and the silicon dioxide layer isetched in seconds while the silicon nitride layer is stable for minutes,whereby to prevent etching of the silicon nitride while the silicondioxide is being etched. Although the hinge is desirably furthersoftened thereby, the removal of the silicon dioxide layer 235 is notrequired.

Although the 5 micrometer thick silicon pad or cantilevered member 16may be considered to be reasonably reflective, it still only has areflectivity of about 30% to the red light typical of lasers in AFM. Toincrease the reflectivity of the silicon pad to on the order of 90%, ametal layer 261 (for example, about 50 nanometers thick layer of gold,or other suitable metal such as aluminum) is preferably depositedthereon. When the sensor has symmetric torsion bars 76, it has beenfound to be immune to the thermal bimorph effect which warps asymmetriccantilevers with gold coatings. Other metals may of course be depositedas suitable, such as, for example, cobalt to produce magneticallysensitive levers for MAC (magnetic-AC) AFM, as discussed in W. Han etal, “A Magnetically Driven Oscillating Probe Microscope for Operation inLiquids,” Applied Physics letters, vol. 69, 1996, pp 4111-4113. Thesensor or oscillator 10 may then be calibrated using principles commonlyknown to those of ordinary skill in the art to which this inventionpertains.

The handle chip edge 25, which is still attached to the wafer 27, issuitably pre-etched, using principles commonly known to those ofordinary skill in the art to which this invention pertains, to allow thehandle chip 14, after it has been formed, to be easily removed (bycontrolled breaking along the pre-etched line 25) from the wafer 27.

While the fabrication process has been discussed with reference to AFMsensor construction and with respect to sensor 10 in FIG. 1, it shouldbe understood that it is also applicable to other designs of sensorsincluding those described herein having torsion bars such as shown inFIGS. 3, 4, 5, 6, and 10 and is also applicable for other applicationssuch as, for example, multi-axis accelerometers, gyroscopes, movablemirrors, switches, magnetometers, and various other kinds ofoscillators.

An alternative wafer 27 that may be used for making an oscillator inaccordance with the present invention is what is known as thesilicon-on-insulator wafer. Such a wafer has a thin layer of silicondioxide sandwiched between two silicon layers of desired thickness. Sucha wafer eliminates the need to monitor the etch-front during the KOHetch, since silicon dioxide may be used as the etch-stop layer for theKOH etch.

It should be noted that a second application Ser. No. 11/506,757, filedAug. 18, 2006, (application publication 2007/0062265) has also beenfiled by the same inventors of the present application containing thesame or substantially the same disclosure.

It should be understood that, while the present invention has beendescribed in detail herein, the invention can be embodied otherwisewithout departing from the principles thereof, and such otherembodiments are meant to come within the scope of the present inventionas defined by the appended claims.

1. A method of making a device which includes an oscillator which isconnected to a support structure by at least one flexible hingecomprising the steps of: (a) depositing on a first material a layer of asecond material; and (b) forming in the first material the supportstructure and the oscillator, including applying an etchant which isselective for the first material to etch all the way through the firstmaterial and leave the second material substantially unetched to therebyform the at least one flexible hinge of the second material.
 2. A methodaccording to claim 1 wherein the forming step includes forming amembrane extending from the support structure, etching the membrane todefine a raised portion, and undercutting the raised portion to form asensing tip.
 3. A method according to claim 1 further comprisingselecting the first material to be silicon and selecting the secondmaterial to be silicon nitride.
 4. A method according to claim 3 furthercomprising forming the oscillator and support structure to each havethickness of at least about 1 micron and depositing the second materiallayer to have thickness less than about 400 nanometers.
 5. A methodaccording to claim 1 further comprising forming the oscillator andsupport structure to each have thickness of at least about 1 micron anddepositing the second material layer to have thickness less than about400 nanometers.
 6. A method according to claim 1 further comprisingdepositing the second material layer to be thinner than either of theoscillator and the support structure.
 7. A method according to claim 1comprising applying the etchant to form the at least one flexible hingeas a pair of co-axial torsion bars.
 8. A method of making a device whichincludes an oscillator which is connected to a support structure by apair of co-axial torsion bars comprising the steps of: (a) depositing ona first material a layer of a second material; and (b) forming in thefirst material the support structure and the oscillator, includingapplying an etchant which is selective for the first material to etchall the way through the first material and leave the second materialsubstantially unetched to thereby form a pair of portions of the secondmaterial layer on the support structure and the oscillator respectivelyand the pair of co-axial torsion bars of the second material connectingthe second material portions.
 9. A method according to claim 8 furthercomprising forming the oscillator for movement over a surface to senseinformation about the surface as part of an atomic force microscope. 10.A method according to claim 8 wherein the forming step includes forminga membrane extending from the support structure, etching the membrane todefine a raised portion, and undercutting the raised portion to form asensing tip.
 11. A method according to claim 8 further comprisingselecting the first material to be silicon and selecting the secondmaterial to be silicon nitride.
 12. A method according to claim 11further comprising forming the oscillator and support structure to eachhave thickness of at least about 1 micron and depositing the secondmaterial layer to have thickness less than about 400 nanometers.
 13. Amethod according to claim 8 further comprising forming the oscillatorand support structure to each have thickness of at least about 1 micronand depositing the second material layer to have thickness less thanabout 400 nanometers.
 14. A method according to claim 8 furthercomprising depositing the second material layer to be thinner thaneither of the oscillator and the support structure.
 15. A method ofmaking a device which includes an oscillator movable over a surface tosense information about the surface as part of an atomic forcemicroscope and which is connected to a support structure by at least oneflexible hinge comprising the steps of: (a) depositing on a firstmaterial a layer of a second material; and (b) forming in the firstmaterial the support structure and the oscillator, including applying anetchant which is selective for the first material to etch all the waythrough the first material and leave the second material substantiallyunetched to thereby form the at least one flexible hinge of the secondmaterial.
 16. A method according to claim 15 comprising applying theetchant to form the at least one flexible hinge as a pair of co-axialtorsion bars.
 17. A method according to claim 15 further comprisingdepositing the second material layer to be thinner than either of theoscillator and the support structure.
 18. A method according to claim 15wherein the forming step includes forming a membrane extending from thesupport structure, etching the membrane to define a raised portion, andundercutting the raised portion to form a sensing tip.
 19. A methodaccording to claim 15 further comprising selecting the first material tobe silicon and selecting the second material to be silicon nitride. 20.A method according to claim 15 further comprising forming the oscillatorand support structure to each have thickness of at least about 1 micronand depositing the second material layer to have thickness less thanabout 400 nanometers.